I dont think it matters (and i dont mean that harshly). The person in the op wants practical advice - squabbling between other posters is really not important in the grand scheme of things. You've made your point, if I were you I'd just leave it otherwise you may detract from the advice being sought.
I think the assumptions niceguy2 made were misogynist, but at the same time I think it would be a stretch to interpret the OP as implying anything other than that the legal father had not initially realised that he was not the biological father. So your "how do you know there wasn't..." is rather far-fetched, but that doesn't alter the fact that there are any number of possible scenarios besides the "lying cheating slapper" option so delicately outlined by niceguy2.
Niceguy2 seems to have also completely missed the legal point that the OP was asking about, mind you, so not an edifying contribution to the thread in general.
i agree professor, my suggestion is a bit of a stretch but i just hate the fact that woman having sex with two people automatically equals =slapper/liar/cheat. as if that is the only possible reason that this man is on the BC of a child that isn't his. i mean, i have seen enough JK (sorry, but i have watched it) to know that some men will agree to their names being added knowing full well there is a chance they aren't the father.
More likely when she got pregnant they were not in an exclusive relationship. I know of more than one situation like this. Admittedly, in the situations I know, it was established pretty firmly and quickly who the child did belong to (in one memorable case because the poor little girl was the SPITTING image of her father! which was not attractive! ) but it's not inconceivable that this woman didn't want to do that at the time. Or bearing in mind how many woman are clueless about how conception works and dates etc, it's entirely possible she genuinely didn't realise who the baby's father was.